Here, the phase stability of high entropy alloy (HEA), Al 0.5TiZrPdCuNi, under fast electron irradiation was studied by in-situ high voltage electron microscopy (HVEM). The initial phase of this alloy quenched from the melt was dependent on cooling rate. At high cooling rates an amorphous phase was obtained, whereas a body-centered cubic ( b.c.c.) phase were obtained at low cooling rates. By thermal crystallization of the amorphous phase b.c.c. phase nano-crystals were formed. Upon fast electron irradiation solid state amorphization (SSA) was observed in b.c.c. phase regardless of the initial microstructure (i.e., “coarse crystalline structure” or “nano-crystalline structure with grain boundaries as a sink for point defects”). SSA behavior in the Al 0.5TiZrPdCuNi HEAs was investigated by in-situ transmission electron microscopy observations. Because the amorphization is very rarely achieved in a solid solution phase under fast electron irradiation in common metallic materials, this result suggests that the Al 0.5TiZrPdCuNi HEA from other common alloys and the other HEAs. The differences in phase stability against the irradiation between the Al 0.5TiZrPdCuNi HEA and the other HEAs were discussed. This is the first experimental evidence of SSA in HEAs stimulated by fast electron irradiation.

@article{osti_1454411,
title = {Solid state amorphization of metastable Al0.5TiZrPdCuNi high entropy alloy investigated by high voltage electron microscopy},
author = {Nagase, Takeshi and Takeuchi, Akira and Amiya, Kenji and Egami, Takeshi},
abstractNote = {Here, the phase stability of high entropy alloy (HEA), Al0.5TiZrPdCuNi, under fast electron irradiation was studied by in-situ high voltage electron microscopy (HVEM). The initial phase of this alloy quenched from the melt was dependent on cooling rate. At high cooling rates an amorphous phase was obtained, whereas a body-centered cubic (b.c.c.) phase were obtained at low cooling rates. By thermal crystallization of the amorphous phase b.c.c. phase nano-crystals were formed. Upon fast electron irradiation solid state amorphization (SSA) was observed in b.c.c. phase regardless of the initial microstructure (i.e., “coarse crystalline structure” or “nano-crystalline structure with grain boundaries as a sink for point defects”). SSA behavior in the Al0.5TiZrPdCuNi HEAs was investigated by in-situ transmission electron microscopy observations. Because the amorphization is very rarely achieved in a solid solution phase under fast electron irradiation in common metallic materials, this result suggests that the Al0.5TiZrPdCuNi HEA from other common alloys and the other HEAs. The differences in phase stability against the irradiation between the Al0.5TiZrPdCuNi HEA and the other HEAs were discussed. This is the first experimental evidence of SSA in HEAs stimulated by fast electron irradiation.},
doi = {10.1016/j.matchemphys.2017.07.071},
journal = {Materials Chemistry and Physics},
number = C,
volume = 210,
place = {United States},
year = {2017},
month = {7}
}

Compression behavior of the Al 0.5CoCrCuFeNi high-entropy alloy (HEA) was studied at different temperatures from 673 K to 873 K at a low strain rate of 5 x 10 –5/s to investigate the temperature effect on the mechanical properties and serration behavior. The face-centered-cubic (fcc) structure is confirmed at the lower temperature of 673 K and 773 K, and a structure of mixed fcc and body-centered cubic (bcc) is identified at a higher temperature of 873 K after compression tests using high-energy synchrotron x-ray diffraction. As a result, by comparing the stress–strain curves at different temperatures, two opposite directions ofmore » serrations types were found, named upward serrations appearing at 673 K and 773 K and downward serrations at 873 K, which may be due to dynamic strain aging.« less

Compression behavior of the Al 0.5CoCrCuFeNi high-entropy alloy (HEA) was studied at different temperatures from 673 K to 873 K at a low strain rate of 5 x 10 –5/s to investigate the temperature effect on the mechanical properties and serration behavior. The face-centered-cubic (fcc) structure is confirmed at the lower temperature of 673 K and 773 K, and a structure of mixed fcc and body-centered cubic (bcc) is identified at a higher temperature of 873 K after compression tests using high-energy synchrotron x-ray diffraction. As a result, by comparing the stress–strain curves at different temperatures, two opposite directions ofmore » serrations types were found, named upward serrations appearing at 673 K and 773 K and downward serrations at 873 K, which may be due to dynamic strain aging.« less

Compression experiments of the Al 0.5CoCrCuFeNi high-entropy alloy (HEA) under displacement control were conducted at different temperatures ranging from 673 K to 873 K with a low strain rate of 2 x 10 -4/s to study its serration behavior. Samples after compression tests were investigated, using the synchrontron-diffraction technique and transmission-electron microscopy. By comparing the stress-strain curves at different temperatures, two opposite directions of serrations were observed, named the upward serration appearing at 573 K and 673 K and the downward serration at 773 K and 873 K. The different directions of serrations were discussed in terms of not onlymore » the relationships among the stress vs. strain, stress vs. time, and strain vs. time, but also the interactions among dislocations, atoms, and nanoparticles. Lastly, the temperature effect on the flow serration is discussed by referring to a theoretical framework for the initiation of the serrated flow. Beyond a critical high temperature, the initiation of the serrated flow becomes swiftly difficult, and ultimately the plastic flow in the full deformation range turns smooth. Such a theoretical prediction of normal behavior is essentially in qualitative agreement with the experimental observation in the present work, i.e., the critical strain to intiate the serration decreases, with the increasing temperature.« less

Li 2Ru 0.5Mn 0.5O 3, a high capacity lithium rich layered cathode material for lithium-ion batteries, was subject to comprehen-sive diagnostic studies including in situ/ex situ X-ray diffraction (XRD), X-ray absorption spectroscopy (XAS), pair distribu-tion function (PDF) and high resolution scanning transmission electron microscopy (STEM) analysis, to understand the cor-relations between transition metal chemistry, structure and lithium storage electrochemical behavior. Ru-Ru dimers have been identified in the as-prepared sample and found to be preserved upon prolonged cycling. Presence of these dimers, which are likely caused by the delocalized nature of 4d electrons, is found to favor the stabilization of themore » structure in a lay-ered phase. The in situ XAS results confirm the participation of oxygen redox into the charge compensation at high charge voltage, and the great flexibility of the covalent bond between Ru and O may provide great reversibility of the global struc-ture despite of the significant local distortion around Ru. In contrast, the local distortion around Mn occurs at low discharge voltage and is accompanied by a “layered to 1T” phase transformation, which is found to be detrimental to the cycle per-formances. It is clear that the changes of local structure around individual transition metal cations respond separately and differently to lithium intercalation/deintercalation. Here, cations with the capability to tolerate the lattice distortion will benefit for maintaining the integrality of the crystal structure and therefore is able to enhance the long-term cycling performance of the electrode materials.« less

Li2Ru0.5Mn0.5O3, a high capacity lithium-rich layered cathode material for lithium-ion batteries, was subject to comprehensive diagnostic studies, including in situ/ex situ X-ray diffraction, X-ray absorption spectroscopy (XAS), pair distribution function, and high resolution scanning transmission electron microscopy analysis, to understand the correlations between transition-metal chemistry, structure, and lithium storage electrochemical behavior. Ru-Ru dimers were identified in the as-prepared sample and found to be preserved upon prolonged cycling. Presence of these dimers, which are likely caused by the delocalized nature of 4d electrons, is found to favor the stabilization of the structure in a layered phase. The in situ XAS resultsmore » confirm the participation of oxygen redox into the charge compensation at high charge voltage, and the great flexibility of the covalent bond between Ru and O may provide great reversibility of the global structure despite the significant local distortion around Ru. In contrast, the local distortion around Mn occurs at low discharge voltage and is accompanied by a layered to 1T phase transformation, which is found to be detrimental to the cycle performances. It is clear that the changes of local structure around individual transition-metal cations respond separately and differently to lithium intercalation/ deintercalation. Cations with the capability to tolerate the lattice distortion will be beneficial for maintaining the integrality of the crystal structure and therefore is able to enhance the long-term cycling performance of the electrode materials.« less